Gravity always pulls matter together. The baseball moves too slowly and gets pulled back to Earth. The spaceship moves fast enough to escape Earth, but it never escapes gravity entirely.

When the word first got out that the expansion of the universe was accelerating, many astronomers questioned the results. They felt that the observations must be wrong, or the interpretation must be flawed. The whole concept was so difficult to believe because it requires significant changes in our understanding of the way the universe works.

Say you step outside and throw a baseball up into the air. The gravity of Earth begins immediately to act on the baseball, slowing it down even as it rises into the air. The upward speed of the baseball slows until it stops at its peak, then gravity's pull causes it to drop down at an ever-increasing speed. What you can't see is that the baseball also has a tiny gravitational pull that acts upon Earth. Gravity always acts to pull matter together.

Now consider a spaceship. If launched with enough speed, a spaceship will escape Earth's gravity to the extent that it will not fall back to the planet. However, it hasn't escaped the pull of Earth entirely. Though it travels away, the spaceship will be continuously slowed — just not to the point where it stops.

Competing Models

These same concepts apply to the expansion of space. That expansion was launched in the Big Bang, and ever since then, each bit of matter in the universe has been attracted to every other bit by the force of gravity. This should have been slowing down the expansion.

Before the discovery of dark energy, scientists had two models of how the universe's expansion would work. In one scenario, there would be enough matter in the universe to slow the expansion to the point where, like the baseball, it would come to a halt and start to retract, everything crashing back together in a "Big Crunch."

In the other scenario, there would be too little matter to stop the expansion and everything would drift on forever, always slowing and slowing but never stopping — like the spaceship. The galaxies would drift apart from each other until they were out of view. The universe would continue growing larger as countless generations of stars faded and died out. It would end in a vast, dark, and cold state: a "Big Chill," if you will.

Though space expands from the energy of the Big Bang, the universe's mass generates enough gravity to eventually stop and reverse the expansion in the Big Crunch scenario (left). In the Big Chill scenario (right), the universe has too little mass and drifts on forever, slowing but never stopping.

Does the Matter Matter?

By the early 1990s, astronomers had calculated how much mass was in the universe, and decided on the Big Chill as the most likely end of the universe. But then dark energy showed up in our observations.

According to the Big Chill, the universe should be expanding more slowly today than it did in the past, because gravity has had time to work on slowing the universe down over all these billions of years. But astronomers found that the universe is moving faster today than it was a billion years ago, meaning something must be working to speed it up.

This result seems crazy because gravity always pulls and slows — it never pushes. Yet some force appears to be pushing the universe apart. Astronomers, concluding that we just don't know what this force is, have attributed it to a mysterious dark energy.

The Big Rip

The universe expands faster and faster, until galaxies and even atoms are eventually torn apart in the Big Rip scenario.

With dark energy, the fate of the universe might go well beyond the Big Chill. In the strangest and most speculative scenario, as the universe expands ever faster, all of gravity's work will be undone. Clusters of galaxies will disband and separate. Then galaxies themselves will be torn apart. The solar system, stars, planets, and even molecules and atoms could be shredded by the ever-faster expansion. The universe that was born in a violent expansion could end with an even more violent expansion called the Big Rip.

So out of the three scenarios for the fate of the universe — re-collapse to a Big Crunch, expand ever more slowly to a Big Chill, or expand ever faster to a Big Rip — we have managed to narrow the possibilities down somewhat.

Evidence has ruled out the Big Crunch. The Big Chill is probably the least that will happen. Whether or not the universe goes all the way to a Big Rip depends on what dark energy really is, and whether it will stay constant forever or fade away as suddenly as it appears to have arisen. And that we do not yet know.

No matter which scenario is right, the universe still has at least a few tens of billions of years left — which leaves us plenty of time to look for the answers.